Bandpass filter with pseudo-elliptic response

ABSTRACT

The invention proposes an H-plane filter with inductive irises which exhibits a quasi-elliptic response while retaining the same compactness as a filter having a Chebyshev response. The invention furthermore makes it possible to use a large number of transmission zeros. For this purpose, there is proposed a waveguide filter comprising at least one cavity 4-delimited by at least two inductive irises. The filter furthermore comprises at least one floating insert  1  placed in one of the inductive irises. The invention is also a process for manufacturing the waveguide filter incorporating at least one insert.

The invention pertains to a bandpass filter with pseudo-ellipticresponse of waveguide type. Such a filter is used in particular inhigh-frequency transmission systems.

The mass-market development of broadband bidirectional transmissiondevices requires the use of a filtering device exhibiting considerableconstraints in terms of selectivity, bandwidth, bulkiness and cost.These constraints are very considerable at the level of the filteringcarried out on the antenna side to isolate transmission and receptionwhere signals lying in two very close bands have to be isolated from oneanother.

Among the filtering technologies usable for millimetre frequencies, thetechnologies of waveguide type exhibit a quality factor high enough tomeet the requirements. The waveguide filters most commonly used arenowadays E-plane filters with dielectric insert and H-plane filters withinductive irises.

Beyond 40 GHz, and for highly selective filters, it is preferable to useH-plane filters with inductive irises. FIG. 1 represents a bandpassfilter of order 3 with four inductive irises possessing a Chebyshev typeresponse. Such a filter, in order to be highly selective, has to have ahigh order N, giving rise to an increase in the number of irises whichis equal to N+1. However, the increase in the number of irises causes anincrease in the size of the filter.

In order to increase the selectivity of an iris filter, it is known, forexample, from the article by W. MENZEL et al, “Planar integratedwaveguide diplexer for low cost millimeter-wave applications” EUMC, pp676-680, September 1997, to introduce transmission zeros near thepassband. The introduction of transmission zeros produces aquasi-elliptic response which improves the selectivity of the filter. Onthe other hand, the introduction of transmission zeros is achieved byadding sections of guide (or resonant cavities) placed perpendicularlyto the principal axis of the filter, therefore rendering the filter lesscompact. Furthermore, the number and the frequency positioning of thetransmission zeros is limited on account of the method ofimplementation.

An aim of the invention is to propose an H-plane filter with inductiveirises which exhibits a quasi-elliptic response while retaining the samecompactness as a filter having a Chebyshev response. A second aim is tobe able to use a large number of transmission zeros. For this purpose,there is proposed a waveguide filter with inductive iris in which atleast one floating insert is placed in an iris.

The invention is a waveguide filter comprising at least one cavitydelimited by at least two inductive irises. The filter furthermorecomprises at least one floating insert placed in one of the inductiveirises.

The expression floating insert should be understood to mean a metalinsert that is not electrically linked to the waveguide so that itspotential is floating as a function of the electromagnetic fieldcirculating in the waveguide.

According to various preferred embodiments, the floating insert isplaced nearer to the edge of the iris than to the centre of the iris.The filter comprises at least one block of dielectric foam inside thewaveguide. The floating insert is printed on the block of foam. The foamhas a relative dielectric constant of close to 1.

The invention is also a process for manufacturing a waveguide filter inwhich a waveguide is made in two parts, the waveguide comprising atleast one cavity delimited by two irises. Before assembling the twoparts of the waveguide, at least one block of dielectric foam is placedinside the waveguide. The block supports at least one metallizationwhich forms at least one floating insert.

Preferably, the insert is made by a technique of printing on the foam.

The invention will be better understood, and other features andadvantages will become apparent on reading the description whichfollows, the description making reference to the appended drawings inwhich:

FIG. 1 represents an iris waveguide filter according to the state of theart,

FIG. 2 represents various possibilities of embodiment of a floatinginsert in an iris,

FIG. 3 represents an exemplary embodiment of a waveguide filterfurnished with a floating insert,

FIG. 4 represents an exemplary frequency response of the filter of FIG.3,

FIGS. 5 and 6 represent two exemplary embodiments of waveguide filterswith two inserts, according to the invention,

FIGS. 7 and 8 represent two exemplary frequency responses of the filtersof FIGS. 5 and 6,

FIG. 9 illustrates a mode of manufacturing a filter according to theinvention.

FIG. 2 a represents a metal insert 1 placed in an iris delimited by twoshims 2 and 3. The metal insert 1 is placed in a floating manner, thatis to say it does not touch any edge of the waveguide so as to be ableto resonate at a frequency which depends on its length and on thecoupling with the electric field. The coupling with the electric fielddepends among other things on the position of the insert with respect tothe centre of the waveguide and the inclination of the insert withrespect to the axis of the guide. There is at present no computationalmodel for determining the resonant frequency of an insert placed in aniris.

The method used for dimensioning the insert consists in starting from aninsert length equal to λ_(r)/2, with λ_(r) the wavelength correspondingto the desired resonant frequency. Then, with the aid of anelectromagnetic simulator, the resonant frequency is evaluated and thenthe size of the insert is modified as are possibly its inclination andits position in the iris as a function of the result of the simulationperformed. The length of the insert is obtained after a few simulationsand may be further refined with the aid of prototype. If the length ofthe insert is too considerable it is always possible to bend the insertto obtain a C insert (FIG. 2 b), an S insert (FIG. 2 c) or an L insert(FIG. 2 d).

The presence of an insert in a waveguide has the effect of creating atransmission zero for its resonant frequency. The insert transforms asimple guide into a highly selective bandstop filter. A drawback is thatthe insert interacts with the waveguide and produces additionaldisturbances. Placed in a filter, the characteristic of the filter ismodified by the presence of the insert.

FIG. 3 represents, in perspective, a filter furnished with threemutually coupled cavities 4 and with two access paths 6 by way of fouririses 7. The filter of FIG. 3 comprises a floating insert 1 placed inan iris. The filter of FIG. 3 is a filter of the type represented inFIG. 1 so as to have one and the same passband. The floating insert isdetermined in such a way that its resonant frequency is placed outsidethe passband so as to strengthen the rejection of the filter at the bandboundary. The transmission zero being placed at a location where theslope of the filter has to be greatly increased.

In order not to overly disturb the field inside the filter and hence thecharacteristic of the insertless filter, the insert is preferably placedin proximity to a shim 2. It is possible to place the insert at thecentre of the guide, that is to say just where the coefficient ofcoupling with the field is a maximum, but the filter has to beredimensioned accordingly to retain the same passband since tooconsiderable a coupling has the effect of greatly modifying thecharacteristic of the filter and in particular its passband.

FIG. 4 shows a possible exemplary response of the filter of FIG. 3 incomparison with the filter of FIG. 1. The curve 10 corresponds to thefilter of FIG. 1 which has a Chebyshev type frequency response. Thecurve 11 corresponds to the response of the filter of FIG. 3 in the caseof an insert resonating at the frequency 12. The curve 11 corresponds toa pseudo-elliptic type response which exhibits a higher degree ofrejection at the passband upper boundary than a Chebyshev type response.The passband of the filter remains the same.

Of course, the addition of an insert may not be sufficient. Preferably,several inserts are added. FIG. 5 shows a filter with two inserts 50 and51 placed in two different irises. FIG. 6 shows a filter with twoinserts 52 and 53 placed in the same iris. It is entirely possible toplace one, two or more inserts in each iris, in the case of a filterfurnished with four irises, up to eight inserts can be placed, therebymaking it possible to add eight transmission zeros and hence toappreciably strengthen the effect produced at the level of the edges ofthe response of the filter.

When several inserts are used, the size of each insert should bedetermined individually. Then a simulation of the filter is performed,incorporating all the inserts so as to refine the size of the insertsand possibly redimension the shims of the irises.

FIG. 7 shows a response curve 14 of a filter corresponding to FIGS. 5 or6 or for which the resonant frequencies of the inserts are placed on oneand the same side of the passband. Relative to the curve 11, the personskilled in the art may note that the effect produced by the two insertson the curve 14 corresponds to an amplified effect.

FIG. 8 shows a response curve 15 of a filter corresponding to FIGS. 5and 6 and for which the resonant frequencies of the inserts are placedon each side of the passband. Obviously, If one wishes to increase therejection edges on each side of the band, it is possible to resort to amore considerable number of inserts.

The person skilled in the art may note that the bulkiness of a filteraccording to the invention remains unchanged relative to a filter withno transmission zero. Also, the number of transmission zero may be equalto M*(N+1), with M the number of insert per iris and N the order of theiris filter, without thereby changing the bulkiness of the filter.

As far as the making of such a filter is concerned, numerous techniquesare possible. The technique described hereinbelow with the aid of FIG. 9enables such a filter to be made at lesser cost.

A conducting block 90 is moulded and/or machined in order to correspondto a waveguide fitted with shims 91 forming irises. A conducting lid 92serves to close the block 90 thus forming a waveguide filter. First,second and third blocks of foam 93 to 95 are placed in the waveguidebefore closing the lid 92. The blocks of foam 93 to 95 are made forexample from polymethacrylate foam, sold under the trademark ROHACELLHF, and which is for example moulded by thermo-compression. In a generalmanner, the foam used should have a relative dielectric constant ε_(r)of Close to 1, low losses, for example of the order of 10⁻⁴, and onwhich it is possible to make a metallization. The first and the thirdblocks of foam 93 to 95 also serve as substrate for the metal inserts 96and 97. The inserts 96 and 97 are made with the aid of a techniquecompatible with the foam chosen. The metallization is for example adeposition of conducting paint done through a mask on which the patternsto be implanted have previously been inscribed. The paint is for exampleof silver type and should exhibit sufficient mechanical grab to remainon the foam.

Preferably, the entire waveguide is filled with foam so as to obtain ahomogeneous propagation medium. However, it is possible not to fill theentire guide with foam if the behaviour of the foam is much like air. ItIs possible to use for example a single block of foam supporting theinserts, the block being stuck on a side or in the middle of the guide.

Obviously, numerous variants of the invention are possible. The numberof cavity of the filter may vary as a function of the requirements ofthe person skilled in the art. Numerous types of foam may be used. Thechoice of conducting paints is relatively wide. The inserts may be madeaccording to a printing technique other than painting, for example byphotolithography of a metal layer integral with the foam.

1. Waveguide filter comprising at least one cavity delimited by at leasttwo inductive irises, wherein the filter furthermore comprises at leastone floating insert placed in one of the inductive irises and supportedby at least one block of foam.
 2. Filter according to claim 1, whereinthe floating insert is placed nearer to the edge of the iris than to thecentre of the iris.
 3. Filter according to claim 1, wherein the at leastone block of foam is at least one block of dielectric foam inside thewaveguide.
 4. Filter according to claim 3, wherein the floating insertis printed on the block of foam.
 5. Filter according to claim 3, whereinthe foam has a relative dielectric constant of close to
 1. 6. Filteraccording to claim 5, wherein the foam is a polymethacrylate foam. 7.Process for manufacturing a waveguide filter in which a waveguide ismade in two parts the waveguide comprising at least one cavity delimitedby two irises, wherein before assembling the two parts of the waveguide,at least one block of dielectric foam is placed inside the waveguide,and in that the block supports at least one metallization which forms atleast one floating insert.
 8. Process according to claim 7, the insertis made by a technique of printing on the foam.